1. Field of the Invention
The present invention relates to an analog-to-digital converter, for example, an analog-to-digital converter used in a video apparatus such as a digital video camera for converting video signals to digital signals.
2. Description of the Related Art
Various kinds of analog-to-digital converters for converting analog signals to digital signals have been developed in response to demands for higher operating speeds, higher resolution, that is, larger numbers of converted bits, etc. Particularly, in the case of digitization of a video signal, since a high speed, high accuracy, and a resolution of more than 10 bits are required, at the present time flash type and/or sub-ranging type analog-to-digital converters are generally used.
FIG. 1 is a circuit diagram of an example of a flash type analog-to-digital converter. As illustrated, the converter of this example is constituted by 16 cascade-connected resistors R1 to R16 connected between reference voltages VRT and VRB, 15 comparators C1 to C15, a sample and hold circuit 110, and an encoder 108.
The resistors R1 to R16 having the same resistance values generate reference voltages by dividing equally the difference of the reference voltages VRT and VRB and supply the divided reference voltages to the comparators. Note that, for example, the difference of the reference voltages VRT and VRB is set as equal to a full scale voltage of the analog-to-digital converter.
An input analog signal Vin is sampled and held by the sample and hold circuit 110. The held signal is supplied to the comparators. Each of the comparators C1 to C15 compares the level of the reference voltage from each of the connection nodes of the resistors and the analog signal from the sample and hold circuit 110 and outputs binary signal in response to the result of the comparison. For example, if the level of the analog signal is higher than that of the reference voltage, a comparator outputs a high level signal representing the data "1", while if the level of the analog signal is lower than that of the reference voltage, the comparator outputs a low level signal representing the data "0".
In this way, a string of data constituted by a plurality of data "0" and "1" is output by the series of comparators according to the input analog signal Vin. The string of data is input to the encoder 108 and converted to a binary number of predetermined number of bits. In the circuit of FIG. 1, the reference voltages are generated by dividing equally the full scale of the input analog signal Vin into 16 parts, so a 4-bit digital code can be obtained.
Such a flash type analog-to-digital converter converts a digital code of a predetermined number of bits according to the level of the input analog signal Vin. Since a plurality of comparators are used for parallel comparison, the bits of the digital code Dout can be obtained all at once. Therefore, high speed conversion can be achieved. For this reason, this type of converter is called a parallel comparison type or flash type converter. The accuracy of this type analog-to-digital converter is affected by the resistance of the resistors dividing the reference voltages and the fluctuations in the offsets of the comparators.
Another type of analog-to-digital converter used for the digitization of video signals is the subranging type analog-to-digital converter shown in FIG. 2. As illustrated, a sub-ranging analog-to-digital converter is constituted by a sample and hold circuit 110, an analog-to-digital converter (ADC) 120, a digital-to-analog converter (DAC) 130, an adder 140, an ADC 150, and a synthesizer 160.
An input analog signal Vin is sampled and held by the sample and hold circuit 110. The held signal is converted to an m-bit digital code Dm and output to the DAC 130 and the synthesizer 160.
The DAC 130 generates an analog signal Sm having a level changing in response to the input digital code Dm. The adder 140 finds a difference of the held signal of the analog signal Vin from the sample and hold circuit 110 and the analog signal Sm from the DAC 130 and outputs a difference signal Sn.
The ADC 150 converts and outputs an n-bit digital code Dn in response to the level of the difference signal Sn. Then the synthesizer 160 combines the m-bit digital code Dm from the ADC 120 and the n-bit digital code from the ADC 150 and outputs an (m+n)-bit digital code Dout.
In this way, in the sub-ranging type analog-to-digital converter of this example, the ADC 120 outputs the upper m-bit digital code, while the ADC 150 outputs the lower n-bit digital code. The synthesizer 160 combines the upper m bits and the lower n bits and outputs the (m+n)-bit digital code Dout as the final result of the conversion.
The analog-to-digital converters described above, however, suffer from a tradeoff between the operating speed and resolution. There was therefore the problem that a converter capable of satisfying all of the requirements was hard to realize.
For example, a flash type analog-to-digital converter is capable of realizing high speed operation, but requires a plurality of resistors for dividing the voltage and a plurality of comparators. The number of the elements constituting the circuit becomes larger the larger the number of bits and therefore an increase of the layout area was unavoidable. At the present time, actual converters made by integrated circuits (IC) are about 8- to 10-bit types.
Further, the number of the resistors and comparators increases along with the increase of the number of bits. The variations in the elements are also becoming worse. In practice, the deterioration of the accuracy due to variations in the offsets of the comparators has become a bigger problem than that due to variations in the resistances of the resistors. As a result, it is difficult to build a converter of more than 10 bits capacity with a high yield.
In a sub-ranging type analog-to-digital converter, the ADC 120 that outputs the upper m-bit digital code Dm and the DAC for converting that to an analog signal must have a precision of the final result of (m+n) bits of the conversion. Further, a high precision of the adder for performing the subtraction is also required. For this reason, it is difficult to build a sub-ranging type analog-to-digital converter of more than 10 bits capacity with a high yield.
In recent years, along with the digitalization of video apparatuses, the need for analog-to-digital converters for converting video signals has become greater. Considering the frequency characteristic and the dynamic range of video signals, analog-to-digital converters should have converting speeds of more than 20 MHz and resolutions of more than 10 bits. Furthermore, it is preferable to combine the analog-to-digital conversion and the signal processing. Therefore, there is a demand for an analog-to-digital converter having a converting speed of more than 20 MHz and a resolution of more than 10 bits by complementary metal oxide semiconductor (CMOS) circuits.